ncp3170 ON Semiconductor, ncp3170 Datasheet - Page 16

• Manufacturer Part Number: ncp3170
• Description: Ncp3170 Synchronous Pwm Switching Converter
• Manufacturer: ON Semiconductor

to the ESR of the output capacitor and the capacitance
selected, which can be calculated as shown in Equation 14:
Co
C
F
I
ra
V
frequency of operation. When using ceramic capacitors, the
ESR of the capacitor decreases until the resonant frequency
is reached, at which point the ESR increases; therefore the
ripple voltage might not be what one expected due to the
switching frequency. Further, the method of layout can add
resistance in series with the capacitance, increasing ripple
voltage.
but tends to range from 1 nH to 20 nH, where ceramic
capacitors have the lowest inductance and electrolytic
capacitors have the highest. The calculated contributing
voltage ripple from ESL is shown for the switch on and
switch off below:
D
ESL
F
I
response of the power supply. For the first few microseconds
of a load transient, the output capacitor supplies current to
the load. Once the regulator recognizes a load transient, it
adjusts the duty ratio, but the current slope is limited by the
inductor value.
drops due to the current variation inside the capacitor and the
ESR (neglecting the effect of the ESL).
OUT
PP
SW
SW
V
OUT
ESR_C
V
The main component of the ripple voltage is usually due
The impedance of a capacitor is a function of the
The ESL of capacitors depends on the technology chosen,
The output capacitor is a basic component for fast
During a load step transient, the output voltage initially
ESR
ESR_C
10.89 mV + 3
ESLON
1.84 mV +
V
= Ripple voltage from the capacitor
ESLOFF
0.7 mV +
+ I
+
= Output capacitor ESR
= Output capacitance
= Switching frequency
= Output current
= Ripple current ratio
= Duty ratio
= Capacitor inductance
= Switching frequency
= Peak−to−peak current
ESL
OUT
1 nH @ 1.01 A @ 500 kHz
+
34%
I
1 nH
ra
ESL
PP
D
27.5%
( 1 * D )
( 1 * 27.5% )
F
CO
1.1 A
SW
I
PP
5 mW )
ESR
³
F
)
SW
500 kHz
8
8
³
500 kHz
F
SW
1
1
C
OUT
44 mF
(eq. 14)
(eq. 15)
(eq. 16)
http://onsemi.com
³
16
Co
I
DV
current during the load transient without discharging it. The
voltage drop due to output capacitor discharge is given by
the following equation:
C
D
F
F
I
L
V
V
DV
capacitor bank dominates the transient response. Please note
that DV
other, and the larger of these two voltages will determine the
maximum deviation of the output voltage (neglecting the
effect of the ESL). It is important to note that the converters
frequency response will change when the NCP3170 is
operating in synchronous mode or non−synchronous mode
due to the change in plant response from CCM to DCM. The
effect will be a larger transient voltage excursion when
transitioning from no load to full load quickly.
Input Capacitor Selection
produced during the on time of the upper MOSFET, so it
must have a low ESR to minimize losses and input voltage
ripple. The RMS value of the input ripple current is:
D
Iin
I
which point the input capacitance RMS current is half the
TRAN
TRAN
OUT
SW
CROSS
OUT
DV
133.5 mV +
OUT
IN
OUT
A minimum capacitor value is required to sustain the
In a typical converter design, the ESR of the output
The input capacitor has to sustain the ripple current
The equation reaches its maximum value with D = 0.5 at
RMS
ESR
OUT
OUT
OUT−DIS
_
_
OUT
Iin
1.34 A + 3 A
DV
ESR
DIS
RMS
OUT−ESR
_
+
7.5 mV + 1.5 A
DIS
2
= Output capacitor Equivalent Series
= Voltage deviation of V
= Voltage deviation of V
= Duty ratio
= Input capacitance RMS current
= Load current
= Switching frequency
= Loop cross over frequency
= Output voltage
= Output transient current
= Output capacitance
= Duty ratio
= Output transient current
= Output inductor value
= Input voltage
+ I
2
Resistance
effects of ESR
effects of capacitor discharge
and DV
50 kHz
OUT
( 1.5 )
F
+ I
CROSS
I
2
TRAN
TRAN
OUT_ESR
27.5%
D
4.7 mH
44 mF
2
C
5 mW
( 1 * D ) ³
CO
OUT
L
( 1 * 27.5% )
OUT
are out of phase with each
ESR
500 kHz
12 V * 3.3 V
³
V
OUT
OUT
IN
F
SW
* V
due to the
due to the
OUT
(eq. 17)
(eq. 18)
(eq. 19)
³